The reason I ask is because I got confused while reading, m book seems to say its both. I was reading it before and it said that cyanobacteria is a type of proteobacteria, which is a prokaryote. I mean bacteria is a prokaryote right? Then, why my book is also saying that cyanobacteria is a type of algae??

I even read this online too. Is it both a bacteria (or prokaryote) and algae?? How can it be both, if algae is a eukaryote? this just doesn't seem to make sense. I don't know if it's a typo but I seem to be getting the same thing online too. Anyone know what to refer to this organism as?

(in my book, it specifically mentions as follows: "Cyanobacteria are gram negative phototrophic bacteria that vary in shape, size, and method....")I also read that Cyanobacteria is a blue-green bacteria (aka "blue-green algae)...

...but how is it both bacteria, which is prokaryote, and algae (eukaryote) at same time?? please explain

Algae is a vage term that has not a specific taxonomical meaning. It applies to photosynthetic organisms wich lack the structural complexity of plants, including microbes.

There was a time when fungi and bacteria were included in the plant kingtom. Later on enough differences were discovered as to support their taxonomical splitage. Blue-green algae was the first name given to cianobacteria when the separation among prokariotes and eukariotes was still unclear.

Today, the latin alga and its plural algae is used most for photosynthetic members of the protoctists kingdom, including both unicells and muticells but eukariotes either case.

Cyanobacteria is often grouped with algae as it is a photosynthetic and often single cellular 'plant', though what they are exactly is not all that different from what we often define as alga (green, red, and brown alga specifically) they are different fundamentally.

Endosymbiotic theory states that algae (not including cyanobacteria) contain chloroplasts which did not evolve within the algae itself but were rather consumed by a eukaryote (today's algae) in the form of cyanobacteria, in the same manner many animal cells have come to contain mitochondria. Alga are defined as eukaryotic cells which contain these chloroplasts but did not develop into plants with roots or vascular systems. to put it simply, alga were once consumers which ate cyanobacteria, incorporated it into their cell mechanics and for the most part became producers very much like cyanobacteria, and because they essentially do the same thing (photosynthesize) cyanobacteria and alga are often lumped together in the same homogeneous group.

1) Please note that many such called algae are multicellurar. Some are real big. They all lack the complexity of true vascular plants (Metaphytae) what precludes them from conquering land hábitats but in the form of lichens. However some algae, particularly brown ones (Phaeophytae), display a hig degree of tissue differentiation along whith organs pretty close to roots, stems and leaves.

2) There is evidence supporting multiple endosymbiotic origins of chloroplasts. One of such events led prokariotic photosynthesizers (related to today's cyanobacteria) to become today's chloroplasts of green plants. However it has to be pointed that this evolutionary branch arised whithin green algae (Chlorophytes) . On the other hand, and this is new for me, it is believed that ancient phaeophytes acquired their chloroplasts in the shape of eukariotic photosynthesizer unicells. This is supported by these chloroplasts having not two but four membranes.